import java.util.List;

public class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}

class Solution {
    //解法1:迭代法
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        ListNode dummynode  = new ListNode();
        ListNode cur = dummynode;
        while (list1 != null && list2 != null) {
            if (list1.val <= list2.val) {
                cur.next = list1;
                cur = cur.next;
                list1 = list1.next;
            } else {
                cur.next = list2;
                cur = cur.next;
                list2 = list2.next;
            }
        }
        if (list1 != null) cur.next = list1;
        if (list2 != null) cur.next = list2;
        return dummynode.next;
    }
    //解法2:递归
    public ListNode mergeTwoLists2(ListNode list1, ListNode list2) {
        if (list1 == null) return list2;
        if (list2 == null) return list1;
        if (list1.val < list2.val) {
            list1.next = mergeTwoLists(list1.next,list2);
            return list1;
        } else {
            list2.next = mergeTwoLists(list1,list2.next);
            return list2;
        }
    }
}